1. Field of the Invention
The present invention relates to bathymetric sonars. More particularly the invention pertains to bathymetric sonar systems involving a single swept transmit beam projector system which compensates for ship attitude and positioning changes.
2. Description of the Related Art
Oceanographic measurements play an important role in all aspects of ocean science and engineering. Typical applications in applied marine science and engineering include mapping of pollutant transport, environmental monitoring, searching for pollution sources, national security, target localization, search and rescue, marine geophysics, resource exploration and resource production. Oceanographic measurements are a key component in the development of marine engineering, management, and policies.
In this regard, sonar systems may be used to detect, navigate, track, classify and locate objects in water using sound waves. In military applications, underwater acoustics is used for depth sounding; navigation; ship and submarine detection, ranging, and tracking; underwater communications; guidance and control of weapons; and mine detection. Civilian applications of underwater sound detection systems include depth sounding (bathymetry); bottom topographic mapping; object location; underwater beacons; wave-height measurement; doppler navigation; sub-bottom profiling; underwater imaging for inspection purposes; buried-pipeline location; underwater telemetry and control; diver communications; ship handling and docking aid; anti-stranding alert for ships; current flow measurement; and vessel velocity measurement.
A typical active sonar system includes a transmitter, i.e., a transducer commonly referred to as a source or projector to generate the sound waves, and a receiver, i.e., a transducer commonly referred to as a hydrophone to sense and measure the properties of the reflected echoes including frequency, amplitude and phase. In one type of sonar system called a "Mills Cross" system, the transmitter or projector array is mounted along the keel of a ship and radiates sound and the receiver or hydrophone array is mounted perpendicular to the transmitter array. The receiver array receives the echoes of the transmitted sound pulse, i.e., returns of the sound waves generated by the transmitter array. Thus, in a typical sonar system, acoustic energy is generated by the transmitter array, travels to the target, is reflected, and returns to the receiver array which measures the return signal. Such a conventional sonar system and transmitter and receiver array configuration is disclosed in U.S. Pat. No. 3,114,631. In those instances where the transmitter array is mounted along the keel of the ship, the transmitter array projects a fan-shaped sound beam which is narrow in the fore and aft direction but wide athwart ship. The signals received by the hydrophones in the receiver array are summed to form a receive beam which is narrow in the across track but wide in the along track direction. The intersection of the transmit and receive beams define the region in the sea floor from where the echo originated. Typically, by applying different time delays to the different hydrophones signals the receive beams can be steered in different directions and when a number of receive beams are formed simultaneously they, together with the transmit beam, define the sonar geometry.
An important consideration in the operation of sonar systems is the ability to survey an area without gaps in seafloor coverage when employing the system in such applications as depth sounding, bottom topographic mapping, object location, sub-bottom profiling, and underwater imaging for inspection purposes. One significant source of gaps in coverage stems from incomplete compensation of the transmit beam direction for ship attitude and position changes.
U.S. Pat. No. 5,808,967 discloses a two-dimensional array transducer and beamformer and which simultaneously transmits from transducer elements with phase or time-delayed signals to the elements. However, this arrangement uses multiple phase and/or time delay beamformers. Each beamformer has different phase or time delays to simultaneously form multiple beams at different angles of inclination to the array face. Thus multiple, narrow dispersion acoustic beams are formed in two planar dimensions normal to the array face.
U.S. Pat. No. 3,603,920 discloses a doppler effect velocity measuring device for determining the velocity and direction of movement which includes an array of transducer elements and the use of multi-phase signals to provide a plurality of signals of similar frequencies and of differing phase. A signal of a single phase is applied to a group of transducer elements in a cyclical repetition to produce a directive signal from the entire array of transducer elements. U.S. Pat. No. 3,441,904 discloses an electromechanical directional transducer which includes an array of elements having some of the elements wound in reverse phase relative to its neighbors to provide side lobe control. U.S. Pat. No. 4,989,530 discloses a torpedo having a pair of sidemounted planar arrays arranged in row and column means and having seven port and seven starboard beams formed simultaneously. None of these systems provide for projector element and/or receiver element compensation for ship attitude and position changes.
In a typical Mills cross swath bathymetric sonar system, a projector array is mounted along the keel of a ship and a receiver array is mounted perpendicular to the transmitter array. One use of such an array is sector scanning where a number of separate transmit pulses cover an entire swath in the athwartship direction. Such sonar systems can compensate for ship pitch and yaw, but are limited in their ability to compensate in shallow water, where short pulses are typically used. This invention overcomes the shortcomings of sector scanning, by sweeping a beam in the athwart ship direction and compensating continuously for pitch and yaw by using separate frequencies for each projector element. The sweep rate determines the effective pulse width. The faster the sweep, the shorter time the beam is focused on a particular spot on the sea floor, and the shorter the pulse width.